Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (C5H4CH3)Mn(CO)3. Initially marketed as a supplement for use in leaded gasoline, MMT was later used in unleaded gasoline to increase the octane rating. Following the implementation of the Clean Air Act (United States) (CAA) in 1970, MMT continued to be used alongside tetraethyl lead (TEL) in the US as leaded gasoline was phased out (prior to TEL finally being banned from US gasoline in 1995), and was also used in unleaded gasoline until 1977. Ethyl Corporation obtained a waiver from the U.S. EPA (Environmental Protection Agency) in 1995, which allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/L (manganese per liter).[2]

MMT has been used in Canadian gasoline since 1976 (and in numerous other countries for many years) at a concentration up to 8.3 mg Mn/L (though the importation and interprovincial trade of gasoline containing MMT was restricted briefly during the period 1997–1998)[3][4] and was introduced into Australia in 2000. It has been sold under the tradenames HiTEC® 3000, Cestoburn and Ecotane.[5][6]

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Though initially marketed in 1958 as a smoke suppressant for gas turbines, MMT was further developed as an octane improver in 1974. When the United States Environmental Protection Agency (EPA) ordered the phase out of TEL in gasoline in 1973, new fuel additives were sought. TEL has been, and still is, used in certain countries as an additive to increase the octane rating of automotive gasoline.

In 1977, the US Congress amended the CAA to require advance approval by the EPA for the continued use of fuel additives such as MMT, ethanol, ethyl tert-butyl ether (ETBE), etc.[7] The new CAA amendment required a "waiver" to allow use of fuel additives made of any elements other than carbon, hydrogen, oxygen (within certain limits) and nitrogen.[8] To obtain a waiver, the applicant was required to demonstrate that the fuel additive would not lead to a failure of vehicle emission control systems.

Ethyl Corporation (Ethyl) applied to the US EPA for a waiver for MMT in both 1978 and 1981; in both cases the applications were denied because of stated concerns that MMT might damage catalytic converters and increase hydrocarbon emissions. In 1988, Ethyl began a new series of discussions with the EPA to determine a program for developing the necessary data to support a waiver application. In 1990, Ethyl filed its third waiver application prompting an extensive four-year review process. In 1993, the U.S. EPA determined that use of MMT at 8.3 mg Mn/l would not cause, or contribute to, vehicle emission control system failures.[9]

Despite that finding, the EPA ultimately denied the waiver request in 1994 due to uncertainty related to health concerns regarding manganese emissions from the use of MMT.[10]

As a result of this ruling, Ethyl initiated a legal action claiming that the EPA had exceeded its authority by denying the waiver on these grounds. This was upheld by the US Court of Appeals[11] and EPA subsequently granted a waiver which allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/l.

Implementation of this less-toxic alternative to TEL has been controversial. Opposition from automobile manufacturers and some areas of the scientific community has reportedly prompted oil companies to voluntarily stop the use of MMT in some of their countries of operation.[12]

MMT is currently manufactured in the U.S. by the Afton Chemical Corporation, a subsidiary of New Market Corporation.[13] It is also produced and marketed as Cestoburn, by Cestoil Chemical Inc. in Canada.

MMT is a so-called half-sandwich compound, or more specifically a piano-stool complex (since the three CO ligands are like the legs of a piano stool). The manganese atom in MMT is coordinated with three carbonyl groups as well as to the methylcyclopentadienyl ring. These hydrophobic organic ligands make MMT highly lipophilic.

A variety of related complexes are known, including ferrocene, which has also been used as an additive to gasoline.[15]

Although of no practical value, the related compound cyclopentadienyl manganese tricarbonyl (C5H5)Mn(CO)3 is also well studied. Up to two of the CO ligands in MMT can be replaced with thiocarbonyl groups, as illustrated by the compounds (CH3C5H4)Mn(CS)2CO and (CH3C5H4)Mn(CS)(CO)2.

The human and environmental health impacts that may result from the use of MMT will be a function of exposure to either: (1) MMT in its original, unchanged, chemical form and/or (2) manganese combustion products emitted from vehicles operating on gasoline containing MMT as an octane improver.

The general public has minimal to zero direct exposure to MMT as a chemical, before it is combusted in gasoline. As stated by the US EPA in their risk assessment on MMT, "except for accidental or occupational contacts, exposure to MMT itself was not thought likely to pose a significant risk to the general population." Similarly, the Australian National Industrial Chemicals Notification and Assessment Scheme (NICNAS) stated that "[m]inimal public exposure to MMT is likely as a result of spills and splashes of LRP [lead replacement petrol] and aftermarket additives".[16]

The MMT dossier registered in the European Chemical Agency's webpage indicates that before combustion in gasoline, MMT is classified as an acute toxicant by the oral, dermal, and inhalation routes of exposure under the European Union's Classification, Labeling and Packaging Regulation (EC/1272/2008), implementing the Global Harmonized System (GHS) of Classification and Labeling. The US ATSDR (Agency for Toxic Substances and Disease Registry) notes that MMT is very unstable in light and degrades to a mixture of less harmful substances and inorganic manganese in less than 2 minutes.[17][18] Therefore, human exposure to MMT prior to combustion in gasoline would not likely occur at significant levels.

Regarding occupational exposure to the raw concentrated chemical prior to addition in gasoline, it has been noted that acute exposures to high levels of MMT in its raw concentrated form, prior to addition in gasoline, have resulted in giddiness, headache, nausea, chest tightness, dyspnea and paresthesia. In animals, acute lethal exposure to MMT is associated with damage to the lungs, kidney, liver and spleen, as well as tremors, convulsions, dyspnea and weakness. In both animals and humans, slight skin and eye irritation may result from dermal and ocular exposure, respectively. Data show that repeated inhalation exposure to MMT in rats results in histological changes in the lungs at levels greater than 3 mg/m3. No effects were seen in the lungs or brain of monkeys when treated with up to 30 mg/m3 MMT.

Chronic exposure to high levels of manganese, typically in certain occupational activities (such as welding), has also been known to cause manganism, a rare disease with symptoms similar to those of Parkinson's disease. Manganism has generally been eliminated due to proper controls in these occupational settings, such as in the ferro-alloy industry.

The US OSHA (Occupational Health and Safety Administration) has not established a permissible exposure limit specifically for MMT. However, OSHA has set a permissible exposure limit at a ceiling of 5 mg/m3 for manganese and its compounds, while the National Institute for Occupational Safety and Healthrecommends workers not be exposed to more than 0.2 mg/m3, over an eight-hour time-weighted average.[19] In Europe, the MMT DNELs (Derived No Effect Level) for workers by the inhalation and dermal routes of exposure are 0.6 mg/m3 and 0.11 mg/kg-day, respectively. The MMT DNELs for the general population by the inhalation and dermal routes of exposure are 0.11 mg/m3 and 0.062 mg/kg-day, respectively.

In summary, based on the low potential for MMT release under normal storage and use, and MMT's rapid photo-degradation properties, environmental exposures are expected to be minimal. This is emphasized again by NICNAS in their conclusions which state that "[u]se of MMT in internal combustion engines as a fuel additive and subsequent degradation through combustion, and its short persistence in the environment, indicate that aquatic and terrestrial organisms are unlikely to be exposed to MMT at or above levels of concern through existing use as an AVSR. A low environmental risk is predicted".

The health hazards associated with manganese compounds (manganese phosphate, manganese sulphate and manganese tetraoxide) emitted from vehicles operating on gasoline containing MMT have been debated for decades. In 1994 (reaffirmed in 1998, 2001 and 2010), Health Canada concluded that "airborne manganese resulting from the combustion of MMT in gasoline powered vehicles is not entering the Canadian environment in quantities or under conditions that may constitute a health risk"[20] and confirmed they were taking no action with respect to MMT. Similarly, the 2003 NICNAS report ruled that the airborne concentrations of manganese as a result of car emissions from vehicles using fuel containing MMT poses no health hazard.

Additional health studies, overseen by the US EPA, were conducted to explain the transport of manganese in the body. The most recent of these studies were published by the Hamner Institutes for Health Sciences from 2007 through 2011 and submitted to the US EPA under the framework of the CAA . They show that manganese is naturally present in the environment and the body's natural mechanisms can handle a wide range of manganese intake via inhalation or ingestion, such that no significant health effects are anticipated from the use of MMT.[21] While the body's natural mechanisms can be overwhelmed if exposures to manganese are very high, the testing confirms that the body can safely handle inhaled manganese at, and well above, levels observed when MMT is used in gasoline. The studies also indicate that inorganic manganese levels that result from use of MMT are safe for the entire population including vulnerable groups such as infants and the elderly. Although the EPA will now formally review the research findings, the US Agency recommended this data as meeting the Clean Air Act's health study's requirements and objectives.

Similar to the above description of human health risks, the environmental risks from MMT combustion products are also expected to be insignificant. In the assessment conducted by NICNAS, it was concluded that "[m]anganese, the principle degradation by-product from combustion of MMT, is naturally occurring and ubiquitous in the environment. It is an essential nutrient of plants and animals. Environmental exposure to Mn compounds will mostly arise through the gaseous phase. Eventually, these will deposit to land and waters. The emission of Mn into the environment from use of fuels containing MMT is unlikely to develop to levels of concern and therefore poses a low risk for terrestrial or aquatic environments."

Based on the low potential for the release of concentrated MMT (before its combustion in gasoline) under normal storage and use, as well as its rapid photo-degradation properties, it has been concluded in multiple technical and global regulatory assessments that significant impacts to human health or the environment from MMT use are not anticipated. NICNAS concluded that there is "low occupational risk associated with MMT" both "for workers involved in formulating and distributing LRP or aftermarket fuel additives and those involved in automotive maintenance". Further, they also concluded that there is a "low risk" to the public from the use of MMT.

Significant human or environmental exposures associated with manganese compounds (manganese phosphate, manganese sulphate and manganese tetraoxide) from the combustion of MMT are not expected. In Health Canada's risk assessment on the health implications of the manganese combustion products of MMT, it was concluded that manganese exposures from MMT use are unlikely to pose a risk to health for any sub-group of the population. NICNAS similarly concluded that chronic Mn exposures (from all sources combined) are unlikely to be significantly changed by the use of MMT as a fuel additive.

In 2013, a risk assessment on MMT was developed by ARCADIS Consulting and verified by an independent panel, according to the methodology provided by the European Commission in compliance with the requirements of the European Fuel Quality Directive (2009/30/EC). The conclusions of a risk assessment are that "for MMT and its transformation products, when MMT is used as a fuel additive in petrol, no significant human health or environmental concerns related to exposure to either MMT or its transformation [combustion] products (manganese phosphate, manganese sulphate and manganese tetroxide) were identified at use at levels up to 18 mg Mn/l. Depending on the regional needs and the vehicle emission control technology available, an MMT treat rate in the range of 8.3 mg Mn/l to 18 mg Mn/l is scientifically justified and may deliver both environmental and economic benefits without significant adverse effects."[22]

Based on the scientific work performed in the last 20 years on MMT and its combustion products, the use of MMT in fuel at levels up to 18 mg/l does not represent a significant risk to human health or the environment.